This invention is concerned with phyllosilicate materials, and with a novel method of treating such materials. It is particularly concerned with a method of hydrating such materials so that they can be delaminated to essentially unit cell dimensions.
Essentially any phyllosilicate, whether of natural or synthetic origin, may serve as a starting material. However, the natural materials are generally of greater interest from an economic standpoint. The silicate minerals of interest include vermiculite, beidellite, nontronite, volchonskoite, saponite, stevensite, hectorite, the smectites, attapulgite, sepiolite, phlogopite and biopyrobole.
Sheet silicates of the mica type are built of two units, viz., a tetrahedral sheet and an octahedral sheet. The former consists of tetrahedra of Si--O linked together to form a hexagonal network such that the bases thereof are coplanar and the apices thereof point in the same direction. This configuration yields a Si:O ratio of 2:5. In contrast, the octahedral sheet is generated through the impingement of two tetrahedral sheets pointing toward each other and crosslinked by the sharing of oxygens by Mg (or Al, Fe) in octahedral coordination. The two octahedral corners not falling in the plane of apical oxygens are occupied by hydroxyl or fluoride ions.
It is possible that a composite sheet formed in this manner will be electrically neutral. In that case, Van der Waals-type forces bond it to the sheets immediately above and below. More commonly, however, an excess negative charge exists due either to ion substitutions, or unoccupied sites (vacancies), or a combination of both. Differences in properties arise from the degree of charge deficiency as well as the location of the excess charge. Charge balance is restored through the uptake of foreign cations in interlayer positions in 12-fold coordination due to hexagonal rings of oxygens located in the sheets above and below.
In order to create a product from vermiculite, it is usually necessary to delaminate the particles. This involves separating the crystals at the interlayer to form high aspect ratio platelets. These may be suspended and subsequently deposited in any desired form, such as a sheet, or otherwise processed.
The silicate layer units in these minerals have a thickness of about 10 Angstrom units, with the main elemental constituents being Mg, Al, Si, and O.sub.2. These silicate layers are separated by an interlayer composed of water molecules associated with cations, such as Mg++, Na+, K+ and H+.
The three layer micas in general, and natural vermiculite in particular, have been extensively studied because of their potential for thermal resistance and electrical insulation. The interest has been heightened considerably with the recent flight from asbestos products.
Many of the phyllosilicates, however, tend to be quite hygroscopic. Various solutions to this problem have been proposed. For example, it is known that adsorbed water molecules and hydroxyl ions may be removed by thermal treatment. This can be very effective, particularly if carried out under reduced pressure. However, there is usually a strong tendency to rehydrate after the material cools and is exposed to ambient conditions.
For certain applications, a large surface area is highly desirable. This necessitates a high degree of delamination. The surface area in square meters per gram (M.sup.2 /gm.) of vermiculite as received is normally less than one. Even after pulverizing to pass a 270 mesh screen, the value is no more than 2-3 M.sup.2 /gm.
It is conventional practice to delaminate the layered silicates by heating to an elevated temperature. This causes the water-containing interlayer to expand and pop open. It has also been proposed to expand vermiculite particles by refluxing in an aqueous solution of a salt such as lithium chloride. Subsequent application of a shearing force causes the crystals to separate at the interlayer and form an aqueous gel.